Gas turbine engines are commonly used to perform work, either as thrust generating engines or as engines providing rotational power via a transmission connected to the output shaft of the gas turbine engine. Even when the engine is operating as a thrust producing engine, there is frequently a need to provide some means of taking power off a high speed shaft being driven by a turbine wheel forming part of the engine. For example, it may be desirable to utilize rotational power to drive a pump or a generator, or the like. Alternatively, in thrust engines of the type known as ducted fan turbojet engines, a fan is rotationally driven as a consequence of engine operation. As is well-known, many gas turbine engines provide very high rotational speeds. In a number of instances, such speeds are excessive in terms of driving other elements as, for example, the rotor of a dynamoelectric machine or a ducted fan. In the case of the former, high rotational rates create large centrifugal forces acting against components of the dynamoelectric machine rotor which tend to cause it to fly apart. If structurally reinforced sufficiently to resist such forces, frequently, efficiency of the dynamoelectric machine is considerably reduced, usually as a result of an undesirable increase in the effective air gap between the rotor and the stator.
In the case of ducted fans, fan tip speed may be of substantial concern in terms of aerodynamic considerations affecting fan efficiency. Consequently, in virtually all ducted fan machines known today, some sort of a speed reduction coupling is established between the high speed shaft of the gas turbine engine and the ducted fan. In the usual case, the speed reduction coupling is in the form of a planetary gear transmission. Not only are these transmissions expensive to fabricate, they also may be of considerable bulk and weight and undesirably increase the size and weight of the engine as well as the cost thereof.
The present invention is directed to providing a speed reducing coupling that may be employed between the high speed shaft of a gas turbine engine and a load which is inexpensive to manufacture, of minimal bulk and weight.